Orthogonal Frequency Division Multiplexing (OFDM), a modulation technology used in 4G communication systems, transmits data via multiple sub-carriers so that the OFDM is able to resist channel frequency-selective fading. In addition, the OFDM is more efficient in computation because of using Fast Fourier Transform and Inverse Fast Fourier Transform (FFT and IFFT).
The OFDM systems have characteristic of slow spectral sidelobe decay, which causes an inter-carrier interference (ICI) existing between neighboring frequency bands. In an Orthogonal Frequency Division Multiple Access (OFDMA) system, a base station will allocate different resource blocks (RBs) to serve users, and each RB comprises several neighboring sub-carriers. When the base station and those users using different RBs transmit data at the same time, the signal quality at the receiving end would be worse once there is a situation of frequency misalignment at that time. This is because that the significant ICI caused by the sidelobe of each sub-carrier could influence the signal quality received at the receiving end.
With the rapid growth of 5G communication system, the wireless communication service providers owning the granted 4G license may urge for solutions to support technical requirements of both 4G and 5G in a single system, that is simultaneously accommodating users of 4G and 5G system. However, there is a big gap between 5G and 4G systems in how the frequency bands are used, for example the difference of waveforms of carrier signals or the difference of the space between carriers. Different spaces between carriers could makes the ICI more significant, so the users of single system serving both new and legacy system often suffer from the effect caused by ICI.